This invention relates generally to hydraulic drive systems and more specifically, to a hydraulically driven vehicle wherein motors on two of the wheels can be optionally powered as needed. Generally, an off-highway vehicle includes a load carrying body supported on both a pair of main traction wheels and a pair of steerable wheels. When operating under heavy load in soft soil conditions, it is desirable to have the steerable wheels powered as well as the main traction wheels. Once the loaded vehicle reaches a hard surfaced road, however, it is better to be able to deactivate the hydraulic drive from the steerable wheels and rely on the main drive wheels for propulsion. With my invention this is possible.
Various hydraulically driven vehicles are known. The U.S. Pat. to Jennings, et al No. (3,736,732) shows an auxiliary drive system for driving the steerable rear wheels of a combine. U.S. Pat. No. 3,997,017 discloses an auxiliary hydrostatic front wheel drive system having fluid pressure actuated clutches to effect a driving combination between the motors and wheels. U.S. Pat. No. 4,072,009 presents the pressure system for driving the wheels of a skid steered front loader. U.S. Pat. No. 4,140,196 shows an auxiliary drive system for optionally powering the steerable wheels of a four-wheel vehicle.
None of the above disclosed systems have the inherent advantages of my invention. A vehicle equipped with my invention does not require the operator to dismount and reposition a wheel-hub mechanism for either engagement or disengagement. More importantly, when the steerable wheels of my invention are operating in the unpowered mode they do not present a drag of several horsepower on the vehicle. This is because the auxiliary motors used in my system do not pump large quantities of fluid around a closed loop when the wheels turn the motors causing them to act as pumps.